1. Field of Invention
The present invention relates to a shallow trench isolation structure and method of manufacture. More particularly, the present invention relates to a method of manufacturing a shallow trench isolation structure having oxide spacers.
2. Description of Related Art
In the conventional fabrication of semiconductor devices, shallow trenches are normally formed between the devices, and then filled by an insulating material to form a field oxide layer for isolation purpose. FIGS. 1A and 1B are cross-sectional views showing the steps in the fabrication of a conventional shallow trench isolation structure. First, as shown in FIG. 1A, a pad oxide layer 10 and a silicon nitride layer 12 are sequentially formed over a semiconductor substrate 1. Then, using conventional photolithographic and etching techniques, a trench isolation structure is formed in the substrate 1. Subsequently, an oxide layer 14 is deposited over the silicon nitride layer 12 and the trench isolation structure. Next, a chemical-mechanical polishing method is used to planarize the structure and to remove unwanted oxide layer 14. Finally, the silicon nitride layer 12 and the pad oxide layer 10 is removed to form the device field oxide layer as shown in FIG. 1B.
In the aforementioned method, the pad oxide layer 10 is removed using a wet etching method that uses hydrofluoric acid as an etchant in the etching process. Using an isotropic etching solution such as hydrofluoric acid, the upper surfaces of the oxide layer 14 adjacent to the substrate 1 can be easily over-etched to form recesses 16 at the upper part of the trench sidewalls.
Furthermore, after the formation of the field oxide layer, a protective sacrificial oxide layer is normally formed over the substrate surface just before any N-well or P-well implantation. In subsequent processes, the sacrificial oxide layer is also similarly removed using hydrofluoric acid. Therefore, the surfaces of the field oxide layer neighboring the substrate may similarly be over-etched.
When the manufacturing processes for forming the semiconductor devices are complete, the recesses on the surface of the field oxide layer adjacent to the substrate surface due to over-etching will lead to an accumulation of electric charges. This can adversely affect the threshold voltage of the device and generates abnormal subthreshold current, which is also known as the kink effect. The lowering of the threshold voltage accompanied by an abnormal subthreshold current will lower the quality of the device, and will lead to a lower product yield. Thus, the outcome of this semiconductor manufacturing process is undesirable.
In light of the foregoing, there is a need in the art to improve the shallow trench isolation structure and its method of manufacture.